When sheet builders are adding sprites to a sheet, they work left to right along each row. They reserve height for the highest sprite seen along that row, resetting the height reservation when the row runs out of space and it moves down to the next row.
As the SpriteCache adds the sprites in a giant batch, it can optimise this operation by ordering the sprites by their height. This reduces wastage where shorter sprites don't use the the full height reserved within the row. The reduced wastage can help the sheet builder allocate fewer sheets, improving load times and improving GPU memory usage as less texture memory is required.
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